Introduction

Following the dieselgate scandal in the fall of 2015, newspapers in Belgium covered nitrogen dioxides for weeks. These molecules are produced during high temperature combustion, like in a car’s motor running on fossil fuels, but are produced in power stations and home central heating systems as well. Nitrogen dioxides are a mixture of nitrogen dioxide (NO2) and nitric oxide (NO). NO does not pose that much of a health risk, because it is short-lived in the atmosphere, where it reacts rapidly with oxygen and ozon, forming NO2. NO2 ,on the other hand, has significant effects on the environment (formation of smog and acid rain) and health problems (respiratory irritation, worsening of asthma or chronic bronchitis).

In Flanders, approximately half of the total NO2 emission is caused by road traffic, where it should be noted that diesel cars produce a lot more NO2 than gasoline cars. As such, NO2 is an important indicator compound for measuring traffic related air pollution. In lots of places in Northern Italy, the Ruhr district (D), Flanders (B) and the south of Holland, exceedances of the European air quality standard for nitrogen dioxide concentration in ambient air occur. Antwerp is a hotspot in Flanders, having high NO2 concentrations, caused by different busy main roads crossing the city region. It stands beyond doubt that its ring road (R1) is the most important source of NO2 in the Antwerp region.
A common method to measure NO2 in air are so-called Palmes diffusion tubes. These tubes were used during the citizen science project CurieuzeNeuzen (a wordplay on Antwerp dialect that would roughly translate to ‘nosing around’ or ‘nosy parkers’) to map NO2 concentrations in the ambient air of the streets of Antwerp and to research the influence of road traffic on the city’s air quality. In the following paragraphs, the method of measurement and the project approach will be discussed in detail.

CurieuzeNeuzen sampled NO2, a good indicator for traffic related air pollution.

Using Palmes diffusion tubes

The Palmes diffusion tube used for sampling nitrogen dioxide.

The monitoring system consists of a small plastic tube, 7 – 10 cm long, with one closed (black) and one open end (yellow, with removable cap). The closed end contains two pieces of gauze (grids), saturated with an absorbent that collects nitrogen dioxide from the surrounding air. The gauze filters are doused with a 50% solution of the absorbent TEA (triethanolamine) in acetone before they are fit in the tubes. The TEA will convert the absorbed NO2 molecules into nitrite, and then bind these ions in complexes. The saturated and dried gauze will be referred to from now on as ‘gel’.

The tubes function using the principle of molecular diffusion, whereby the NO2 gas diffuses spontaneously from an area of high concentration to an area of low concentration. As the absorption plate at the closed, upper end of the tube keeps the NO2 concentration there at zero, NO2 will diffuse into the tube continuously, at a rate directly proportional to the difference in NO2 concentration between the air in the tube and that immediately surrounding it (Fick’s law of diffusion). In this way, Palmes diffusion tubes absorb NO2 from the surrounding air without using an external energy source (suction or active air movement), which is also referred to as passive air sampling. The monitoring tubes are mounted with the open end facing downwards for a sample period of one week up to several months. They can be used for monitoring both inside and outside air quality. After the sampling period is over, the tubes are resealed and sent to a certified lab for analysis.

Why use Palmes diffusion tubes?

Because of their low price and ease of use, it’s possible to use tubes of this type for air quality measurements in large numbers and in large areas. In many cases they serve as a complementary monitoring method, alongside more expensive and precise measurement instruments; those results can be linked to those of the Palmes tubes. However, 2 important shortcomings come with the use of the diffusion tubes. Firstly, they provide only a qualitative value for the NO2 level. Their results will always have to be linked to those of more precise measuring instruments (see also the next section). Secondly, the results provide an average value of the NO2 concentration over a relatively large measurement period. Consequently, they cannot be compared directly with the often used results of instruments measuring for a shorter time period of several minutes up to a few hours. However, they are very useful to qualitatively identify high emission zones of NO2, especially for pollution sources such as traffic.

Several studies show that the abovementioned, passive monitoring method is readily comparable to the continuous measurements used in reference methods based on chemiluminescence. The performance values of these tubes have complied since 1986 with the requirements in the European standard NEN-EN-13518 (1, 2 and 3). We can therefore conclude that when linked to a continuous high-quality measurement method, Palmes diffusion tubes are a good monitoring method for measuring air pollution in long-term studies.

Some of the 4000 NO2 samplers used in this citizen science project.

Approach of the citizen science project

The CurieuzeNeuzen experiment is a citizen science project, where the community was actively involved in scientific research. Citizens collected data under the guidance of professional scientists. CurieuzeNeuzen was initiated by the Ringland Academy, a group of experts and committed citizens who support the proposal of reorganizing and covering the Antwerp ring road through scientific advice and research. Scientific guidance was provided by academics from the Vrije Universiteit Brussel (VUB), the Universiteit Antwerpen (UAntwerpen) en the Onderzoeksinstituut voor Arbeid en Samenleving (HIVA – KULeuven). To develop CurieuzeNeuzen, further expertise was brought in (for free) by countless experts from Flemish and foreign research institutes.

Antwerp, the city where measurements were concentrated, is a hotspot in Western Europe, having high NO2 concentrations, although initiatives to improve air quality are being planned. The May 2016 measurement is meant as a baseline measurement. If, after the introduction of a low emission zone or in case Ringland will be built, a new measurement is done, the effect of these policy measures on the metropolitan air quality can be calculated.
The CurieuzeNeuzen project offered 2000 inhabitants, schools, organizations and companies the chance to measure air quality on the street side of their homes, apartments or buildings. A measuring setup as pictured on the right was used, where a measuring board would be attached to a window pane. The board would contain two Palmes diffusion tubes, sampling NO2 selectively from the ambient air.
The board allows the tubes to be hung on a fixed distance from the building, improving the measurement’s standardization. After 4 weeks the tubes were brought to a lab for analysis. The total amount of NO2 collected in the tube’s gel, is a measure of the mean concentration of NO2 in ambient air. To make sure the measurements are correct, two tubes are used and the mean value of those two measurements is used for data analysis. The Palmes diffusion tubes that were used in the CurieuzeNeuzen project, were tested by the VMM (Flanders Environment Agency) and gave very reliable results.

Analysis of the samplers

After the predetermined measurement period, the tubes are sent to a certified laboratory for analysis. Analysis is based on absorption spectrophotometry. The nitrite, complexed with the absorbent TEA, is extracted first into a watery solution. This solution contains a coloring agent which turns purple upon reacting with the extracted nitrite. The color intensity depends on the released amount of nitrite, and consequently on the amount of absorbed nitrogen dioxide during the measurement period. The color intensity is determined using absorption spectrophotometry, a technique that uses visible or ultraviolet light; the concentration of a solution is measured using the absorption of this light when directed through a sample.

Processing the results

Among the variables taken into account when processing the results are: the average value of two measurement points per location, differences, distribution, street side orientation, building level, street canyon index, and so on… Read on for more about the results.